Optical memory technology that uses optical disks that have pit-shaped patterns as high-density, large-volume memory media is gradually being applied widely to and entering general use in digital audio disks, video disks, document file disks and also data files. Thus, the functions for using a minutely narrowed light beam to successfully achieve recording onto and reproduction of information from an optical disk with high reliability, are divided into three main functions, that is, a focusing function which forms a minute spot at the diffraction limit on the optical disk, focus control (“focus servo”) and tracking control of the optical system, and pit signal (“information signal”) detection.
With recent advances in optical system design technology and the shortening of the wavelengths of the semiconductor lasers serving as light sources, the development of optical disks containing volumes of memory at greater than conventional densities is progressing. As an approach towards higher densities, problems such as an increase in the aberrations due to slanting of the light axis (what is known as “tilt”) were found when investigating an increase in the optical disk side numerical aperture (NA) of the focusing optical system that focuses light beams onto the optical disk. That is to say, the amount of aberration which occurs with respect to the tilt increases when the NA is increased. It is possible to prevent this by thinning down the thickness (substrate thickness) of the transparent substrate of the optical disk.
The substrate thickness of a Compact Disc (CD), which can be considered a first generation optical disk, is approximately 1.2 mm, and the optical head device for CDs uses a light source emitting infrared light (with a wavelength λ3 that is 780 nm to 820 nm, with 800 nm as standard) and an objective lens with an NA of 0.45. Furthermore, the substrate thickness of a Digital Versatile Disc (DVD), which can be considered a second generation optical disk, is approximately 0.6 mm, and the optical head device for DVDs uses a light source emitting red light (with a wavelength λ2 that is 630 nm to 680 nm, with 650 nm as standard), and an objective lens with an NA of 0.6. Moreover, the substrate thickness of a third generation optical disk is approximately 0.1 mm, and the optical head device for these disks uses a light source emitting blue light (with a wavelength λ1 that is 390 nm to 415 nm, with 405 nm as standard), and an objective lens with an NA of 0.85.
It should be noted that in this specification, “substrate thickness” refers to the thickness from a surface of the optical disk (or the optical recording medium) on which the light beam is incident, to the information recording surface. As described above, the substrate thickness of the transparent substrate of the high-density optical disks is set to be thin. From the view point of economy and the space that is occupied by the device, it is preferable that an optical information apparatus can record and reproduce information from a plurality of optical disks having differing substrate thicknesses and recording densities. However for this, it is necessary to have an optical head device provided with a focusing optical system capable of focusing a light beam up to the diffraction limit onto a plurality of optical disks having differing substrate thicknesses.
Furthermore, if diffracting optical elements are used as the optical elements for constituting optical head devices, instead of the refracting optical elements such as lenses and prisms that are usually used, then the optical head devices can be made smaller, slimmer, and lighter.
Diffraction optical elements are optical elements that function by effectively utilizing the diffraction effect of light, and are characterized by corrugations of a depth that is in the order of the wavelength, or by having a refractive index distribution or amplitude distribution that is formed periodically or quasi-periodically on the surface. It is known in the art that if the period of the diffraction optical element is sufficiently large compared to the wavelength, then it is possible to raise the diffraction efficiency to substantially 100% by making the cross section saw tooth-shaped.
However, if the frequency is sufficiently large compared to the wavelength, then the diffracting efficiency of the diffraction optical element reaches 100% only with respect to the design wavelength. Generally, the diffraction efficiency steadily decreases as the wavelength diverges from its design value. Consequently, if diffraction optical elements are used in optical head devices in which light sources of a plurality of wavelengths are mounted so as to handle a plurality of varieties of optical disks, then the diffraction optical elements need to be optimally designed for each wavelength and disposed only in the light path of the wavelength thereof in order to raise the light utilization ratio.
A configuration whose object is to provide an optical head with high light usage efficiency, which has a light source of a plurality of wavelengths and diffraction optical elements that can handle a plurality of different varieties of information recording media, is disclosed in JP 2001-60336A (first conventional example). The first conventional example is described below with reference to FIG. 10.
FIG. 10 is a lateral view of the basic configuration and the state of light transmission of an optical head device according to the first conventional example. As shown in FIG. 10, in the optical head device of the first conventional example, a collimator lens 71 and an objective lens 18 are disposed in the light path from a laser light source 105 to the information recording medium such as a high density optical disk 9 or optical disk 11 such as a CD. The laser light source 105 is a light source that can selectively emit a first light beam of a first wavelength λ1, and a third light beam of a third wavelength λ3, which has a wavelength substantially twice that wavelength. It should be noted that in the description below, wavelengths in the region of 660 nm are also considered, so that these are described as “second wavelengths”. A laser light 205 that is emitted from the semiconductor laser light source 105 is converted to substantially parallel light by the collimator lens 71 after which its light axis is bent by a mirror 20. The light beam 205 whose light axis was bent by the mirror 20 is focused by the objective lens 18 onto the optical disk 9 or 11. The first wavelength λ1 of the first light beam that is emitted by the laser light source 105 satisfies, for example, the relationship 350 nm≦λ1≦440 nm, and its focal spot can be brought to a minute point by provision of the laser light source 105 that emits the first light beam of the first wavelength λ1. Furthermore, the third wavelength λ3 of the third light beam that is emitted by the laser light source 105 satisfies, for example, the relationship 760 nm≦λ3≦880 nm, and optical disks such as CDs and CD-Rs can be read out by provision of the laser light source 105 that emits the third light beam of the third wavelength λ3. In this manner, in the optical head device of the first conventional example, the wavelength of the light that is emitted is determined according to the type of optical disk that is to be read out, and a light beam of that wavelength is emitted selectively.
Furthermore, in the optical head device of the first conventional example, a diffraction optical element 85 is disposed in the light path between the mirror 20, which bends the light axis, and the objective lens 18, for the purpose of correcting chromatic aberrations of the objective lens 18. Here, the objective lens 18 and the collimator lens 71 are aspherical lenses.
As described above, the diffraction optical element generally shows a high diffraction efficiency with respect to the design wavelength, but the diffraction efficiency gradually decreases as it diverges from this. Consequently, when the diffraction optical element is disposed in the light path is passed by both the light beam of the design wavelength and light beams other than this, the diffraction efficiency deteriorates with respect to one of the wavelengths.
However, if the period of the diffraction optical grating is sufficiently large compared to the wavelength, then when the wavelength is approximately half the design wavelength, the first order diffraction efficiency is substantially 0, but the second order diffraction efficiency is exceptionally high at substantially 100%.
In the first conventional example, an optical head device is disclosed in which, in a two wavelength optical head device that is capable of handling both high density optical disks that use a blue light source and optical disks such as CDs and CD-Rs, setting the relationship of the wavelength size of the two wavelengths to be approximately double (in actual fact, it is in the order of 1.8 to 2.1), by principally emitting second order diffracted light from the diffraction optical element 85 when handling the high density optical disks (when using the first light beam of the first wavelength λ1) and by principally emitting first order diffracted light from the diffraction optical element 85 when handling optical disks such as CDs and CD-Rs (when using the third light beam of the third wavelength λ3), then a high diffraction efficiency can be obtained with respect to either wavelength even if the diffraction optical element 85 is disposed in the same light path, and as a result, an optical head device that is capable of achieving excellent optical characteristics, is attained.
Furthermore, a diffraction angle of the diffraction optical element is determined by the wavelength, the frequency and the diffraction order, however in the first conventional example, by using mainly second order diffracted light at the first wavelength λ1, and using mainly first order diffraction light at the third wavelength λ3, which has a wavelength substantially twice as long, the same diffracting angle can be set, even if the wavelength differs.
The cross-section of the diffraction optical element is substantially saw tooth-shaped. In the case of a transparent-type element in the first conventional example, the depth h of the saw tooth-shape is set such that it is practically within the range from h1=2λ1/(n−1) to h3=λ3/(n−1) with respect to the first wavelength λ1, the third wavelength λ3 and the refractive index n of the material of the diffraction optical element 85, such that the diffraction efficiency is large for all of the wavelengths. For example, if λ1=400 nm, λ3=800 nm and n=1.5, then because h1=h3, with the transparent type element, h=1.6 μm.
Moreover, in the first conventional example, a case is also disclosed in which DVDs, which are optical disks of higher density than CDs, can be interchangeably recorded and reproduced by also providing a laser light source that emits a light beam of a second wavelength λ2, which has a wavelength substantially 1.5 times that of the light beam of the first wavelength λ1. In this case, a single, or a plurality of diffraction optical elements are disposed in the light path of the three wavelength light beam. The diffraction optical element principally emits sixth order diffraction light with respect to the light beam of the first wavelength λ1, principally emits third order diffraction light with respect to the light beam of the third wavelength λ3, and principally emits fourth order diffraction light with respect to the light beam of the second wavelength λ2.
In the first conventional example, it seems that the second wavelength λ2 that is capable of recording and reproducing DVDs satisfies the relationship 570 nm≦λ2≦680 nm. However, from the ease of manufacture of semiconductor laser light sources, it is preferable that the second wavelength λ2 is set to 650 nm to 680 nm, and in actual commercially available DVD optical information apparatuses, wavelengths of 650 nm to 680 nm are used, with 660 nm as the standard.
Furthermore, due to the ease of manufacture of the semiconductor lasers it is also preferable that the first wavelength λ1 for optical disks of an even higher density than next generation DVDs is set to 400 nm to 410 nm, with 405 nm as the standard.
Using laser light sources having the first wavelength λ1 and the second wavelength λ2, it is useful to use diffraction optical elements for correcting chromatic aberrations and the like, even in optical systems in which DVDs, and optical disks of a higher density than the next generation DVDs are recorded and reproduced.
BK7 glass is widely used as a material for the diffraction optical element. The refractive index n1 of BK7 is approximately 1.5302 with respect to the first light beam of the first wavelength λ1=405 nm.
Setting the cross-section grating shape of the diffraction optical element to be saw tooth-shaped, in order to achieve a diffraction grating whose second order diffraction efficiency is substantially 100%, as in the first conventional example, the depth h of the saw tooth shape (the height of the saw tooth) is:h=2λ1/(n1−1)=1530 nm.
Furthermore, the refractive index n2 of BK7 is approximately 1.5142 with respect to the second light beam of the second wavelength λ2=660 nm. Thus, the light path difference that the depth of the saw tooth shape (the height of the saw tooth) h applies to the second light beam of the second wavelength λ2 is:
      h    ⁡          (              n2        -        1            )        ⁢          =            786      ⁢                          ⁢      nm        ⁢                  =          1.19      ⁢                          ⁢      λ2      Thus, because the light path difference that the depth of the saw tooth shape (the height of the saw tooth) h applies to the second light beam of the second wavelength λ2 is not an integer multiple of the second wavelength λ2, the second order diffraction efficiency decreases, and even the first order diffraction efficiency is about 80%.
Setting the cross-section grating shape of the diffraction optical element to be saw tooth-shaped, in order to achieve a diffraction grating whose sixth order diffraction efficiency is substantially 100%, as in a further embodiment disclosed according to the first conventional example, the depth h of the saw tooth shape (the height of the saw tooth) is:h=6λ1/(n1−1)=4580 nm.Thus, the light path difference that the depth of the saw tooth shape (the height of the saw tooth) h applies to the second light beam of the second wavelength λ2 is:
      h    ⁡          (              n2        -        1            )        ⁢          =            2357      ⁢                          ⁢      nm        ⁢                  =          3.57      ⁢                          ⁢              λ2        .            In this manner, because the light path difference that the depth of the saw tooth shape (the height of the saw tooth) h applies to the second light beam of the second wavelength λ2 is not an integer multiple of the second wavelength λ2, the sixth order diffraction efficiency decreases and even the third order diffraction efficiency and the fourth order diffraction efficiency are lower than 60%. Furthermore, the loss becomes a scattered light component, and it is impossible to deny that this is a cause of degradation in signal quality. Moreover, even if the material is changed, there is not a big difference in scattering characteristics, so that even if a different material is selected, it cannot be expected that there will be a noticeable improvement.
Thus, as given above, the first conventional example has a problem in that the light usage efficiency is low when the second light beam of the second wavelength λ2 is used when interchanging DVDs.